The present invention relates generally to equalization techniques for digital modulation such as QAM (quadrature amplitude modulation), and more specifically to a matched filter receiver used in combination with a decision feedback equalizer.
In digital radio transmission systems, intersymbol interference can be equalized using a decision feedback equalizer if the ratio of undesired-to-desired signal (p) is smaller than 1. However, if the desired, direct signal arrives at a receiver later than a undesired, reflected version of the transmitted signal, interference occurs between them, causing what is called multipath fading. When this occurs, the amplitude of the undesired signal is stronger than the desired signal, hence p&gt;1, giving rise to a large negative peak at time t=-T (where T is the symbol timing) with respect to the center, positive peak at the origin of the impulse response of the transmission system, whereas, when p=0, i.e., in the absence of the undesired component, the impulse response is symmetrical with respect to the origin. Under such circumstances, intersymbol interference cannot completely be equalized.
It is known that the transfer function of an optimum filter is the complex conjugate of the spectrum of the input signal and such a filter is called a matched filter. Since the impulse response of the matched filter is a timereversed and delayed version of the input signal, the application of a signal whose U/D ratio is smaller than 1 to the matched filter results in the generation of two negative peaks, one at time t=-T and the other at t=T, both having amplitudes one-half of the amplitude of the original negative peak at time t=-T.
A proposal has been made for using a matched filter in combination with a decision feedback equalizer in order to take advantage of the unique characteristic of the matched filter for equalizing signals affected by multipath fading since the two negative peaks at the output of the matched filter can be easily equalized by the decision feedback equalizer. More specifically, the matched filter is implemented with a transversal filter and a tap-weight controller to control the tap weights of the transversal filter in an adaptive fashion in response to the output of the decision feedback equalizer. The tap-weight controller comprises a plurality of correlation circuits corresponding respectively to the tap-weight multipliers of the transversal filter to detect correlations between delayed incoming signals with the output of the decision feedback equalizer. In each correlation circuit, the detected correlation is integrated over a period L.times.T and divided over time L to produce a tap-weight control signal which is an average value of the integrated signal. Since this control signal is updated at L.times.T intervals, the prior art matched filter cannot equalize signals affected by multipath fading which varies at rates higher than the rate at which the output of the matched filter is updated.